I think you will find that the Social Security Administration is required by federal law to invest any annual SURPLUS into the so-called 'trust fund', which consist of Treasury Notes. As of the end of 2011, the Social Security 'trust fund' contained $2.7 trillion and was drawing 4.4% annual interest. This amount is expected to cover all projected benefit obligations thru 2033, at which point it actually would have to start paying out current benefits from current tax receipts. Until then there will be a surplus in the 'trust fund'. In other words, for at least the next 20 years, there is no way in hell that you could describe Social Security as a 'Ponzi' scheme.
http://en.wikipedia.org/wiki/Social_Security_Trust_Fund